WM8918 datasheet(44/176 Pages) CIRRUS

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WM8918

Production Data

PD, Rev 4.1, January 2012

HIGH PASS FILTER

A digital high pass filter is applied by default to the DMIC path to remove low frequency noise in voice

applications (e.g. wind noise or mechanical vibration). This filter is controlled using the DMIC_HPF

and DMIC_HPF_CUT register bits.

In hi-fi mode the high pass filter is optimised for removing DC offsets without degrading the bass

response and has a cut-off frequency of 3.7Hz at fs=44.1kHz.

In voice mode the high pass filter is optimised for voice communication and it is recommended to

program the cut-off frequency below 300Hz (e.g. DMIC_HPF_CUT=11 at fs=8kHz or

DMIC_HPF_CUT=10 at fs=16kHz).

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R38 (26h)

DMIC Digital

6:5

DMIC_HPF_C

UT [1:0]

DMIC Digital High Pass Filter Cut-Off

Frequency (fc)

00 = Hi-fi mode (fc=4Hz at fs=48kHz)

01 = Voice mode 1 (fc=127Hz at

fs=16kHz)

10 = Voice mode 2 (fc=130Hz at fs=8kHz)

11 = Voice mode 3 (fc=267Hz at fs=8kHz)

(Note: fc scales with sample rate. See

Table 12 for cut-off frequencies at all

supported sample rates)

DMIC_HPF

DMIC Digital High Pass Filter Enable

0 = Disabled

1 = Enabled

Table 11 DMIC Digital 0 Register

Sample

Frequency

(kHz)

CUT-OFF FREQUENCY (Hz)

DMIC_HPF_CUT

=00

DMIC_HPF_CUT

=01

DMIC_HPF_CUT

=10

DMIC_HPF_CUT

=11

8.000

0.7

130

267

11.025

0.9

178

367

16.000

1.3

127

258

532

22.050

1.9

175

354

733

24.000

2.0

190

386

798

32.000

2.7

253

514

1063

44.100

3.7

348

707

1464

48.000

4.0

379

770

1594

Table 12 DMIC High Pass Filter Cut-Off Frequencies

The high pass filter characteristics are shown in the “Digital Filter Characteristics” section.

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